Question
Bacillus cereus
Answer
Bacillus cereus is an aerobic, spore-forming, Gram-positive bacillus, ubiquitous in soil and environment, notable as a cause of food poisoning via two distinct toxin-mediated syndromes.
Two clinical syndromes:
- Emetic (vomiting) syndrome — caused by a pre-formed, heat-stable emetic toxin (cereulide) in improperly stored, reheated cooked rice (a classically associated food, since B. cereus spores survive initial cooking and germinate/produce toxin if the rice is then left at room temperature before reheating); short incubation period (1–5 hours), presenting with nausea and vomiting, resembling staphylococcal food poisoning.
- Diarrhoeal syndrome — caused by a heat-labile enterotoxin produced in the gut after ingestion of contaminated food (meat dishes, sauces, vegetables); longer incubation period (8–16 hours), presenting with watery diarrhoea and abdominal cramps, resembling Clostridium perfringens food poisoning.
Other clinical significance: B. cereus can also cause post-traumatic/post-surgical eye infections (endophthalmitis), particularly after penetrating eye trauma with soil contamination, which can be rapidly destructive to vision; and opportunistic infections (bacteraemia, wound infections) in immunocompromised patients or those with indwelling catheters.
Laboratory diagnosis: culture of the suspected food or stool on selective media (e.g., mannitol-egg yolk-polymyxin agar); demonstration of large numbers of the organism in the implicated food supports diagnosis of food poisoning, since B. cereus can also be found as a low-level environmental contaminant.
Prevention: proper food storage (rapid cooling and refrigeration of cooked food, avoiding prolonged storage at room temperature) and thorough reheating.